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For me it is unacceptable to think of micropropagation and plant tissue culture as synonyms, just like algebra is not tantamount to mathematics and dynamics is not the same as physics.

So some kind of general definition of PTC is needed, that would suit both micropropagation and e.g. fertilization in vitro, in vitro mutagenesis and selection, cell fusion, genetic transformation, biotechnological production of metabolites and proteins etc.

Perhaps it could sound more or less to this effect:

Plant tissue culture = growing plants or plant fragments (organs, tissues, cells, protoplasts) on artificial media under conditions of sterility. Very often p.t.c. is initiated with a small fragment of a plant (the so called explant) and results in formation of complete, though tiny, plants (plantlets) or unorganized cell masses (calluses, cell suspensions).

DJM 29 June, 2006

Generally I do agree. Something like this is needed. ReoON | +++ 09:01, 29 June 2006 (UTC)

Asexual reproduction Among the most ancient and widespread methods of plant propagation we find scions and layerings, which are really shoots produced by asexual (agamic) reproduction of the [cormofites].

Asexual reproduction determines the aggressiveness of a species in colonizing the nearby environment, with evolutionary advantages because the offspring are all clones of the parent -- that is to say, they are genetically identical. In a lot of multicellular cryptogams (fungi, lichens, bryophytes and pteridophytes) this happens through spores, produced following mitosis and contained in the sporangia, in the cormofites (bryophyta, pteridophyta and spermatophyta) through parts of the body of the plant that, once fallen to earth, are able to take root.

The scion is a portion of a plant (usually a stem or branch) cut immediately under a knot, without leaves, and buried. The result of the operation depends on persuading it to take root. Layering consists of making the buried part of the plant take root while it is still united to the parent plant. The buried branch or stem is surrounded by a mound of earth or well dampened peat in which the new roots will grow. This is therefore a more rapid and efficient method than the scion.

Evidently these methods do not bring genetic changes, but serve only to clone the plant.

Micropropagation The use of plant cells or organs has allowed the researchers to study callus culture. Callus culture involves induction of callus tissue (a callus is an unorganized mass of cells) from various types of explants (it may be a petiole, a leaf, a pollen grain, a bud or an anthurium). The original plant does not need to be destroyed. The explants are disinfected, and placed on a medium of growth. The medium is also sterilized to avoid the rapid growth of bacteria or fungus that might suffocate the growth of the callus. A cell will begin to divide itself if the correct nutrient substances (salts, sugars and vitamins) and traces of plant hormones are present. The cells will begin to form buds, which can be further separated and cultivated for a new cycle of propagation. Limiting the number of in vitro cycles, however, has been reported to minimize plant variation.

Calluses can be obtained both from somatic cells and from sexual cells.

Through the passage to protoplasm, the cells fabric-specifications of the plant differentiation in totipotent cells, able that is to express any part of their genetic information. This characteristic, together with the ability to form crowd you cellular immortal, differentiates the vegetable cells from those animal. Even a single cell of a callus can regenerate a whole organism in one of two principal ways:

somatic embriogenesis: the callus develops into an embryo, which develops into an adult plant. For example, this occurs in the carrot. somatic organogenesis: an organ -- usually a leaf bud -- regenerates from the callus, and subsequently the rest of the plant regenerates from that. For example, this occurs in tobacco. In both cases the growth in vitro happens in the Fitotroms, sterile climate-controlled enclosures where temperature, damp and light-dark cycle are checked and programmable. With the somatic embryogenesis vegetable embryos of certain kinds can be encapsulated (as with carrots, celery and tomatoes) to get artificial seeds. An important part of the process depends on vegetable hormones, whose role has been revealed mainly through analysis of the behavior of cells in the crop; the production of roots is checked by the auxine class of hormones, while the Cytokinines regulate the growth of the buds and therefore of the stem. They can be used as inductors in the scions and layerings.

Evidently, these techniques of micropropagation allow production of many plants, even millions, in just a few months. These techniques also can prevent plants from becoming infected, simply by using meristematic fabrics or withdrawing fabrics in the healthy zones of the plant and cultivating in presence of antibiotics and antivirals.

Somaclonal variability Many of the plants produced through micropropagation are not genetically identical to the original. This is referred to as somaclonal variation. It is yet unclear why some of these these variations take place, though explanations for some cases have been demonstrated and other explanations have been proposed.

The most frequent reason why plants are not identical to their donor is mutation of their genetic material, from the nucleus or otherwise. Frequent mutations occur when the cells are cultivated in vitro. Depending on the goal (obtaining identical plants or, on the contrary, attempting to get new characteristics) somaclonal variability is considered a disadvantage or a benefit.

There is an important distinction between genetic variability (hereditary mutations of the DNA) and non-hereditary epigenetic variability.

Obviously not all the mutations will be advantageous, but among the many that happen some will be the desired ones. As it is unlikely that the same mutation will happen on both alleles of a diploid plant (not to mention tetraploid plants), when one wants to exploit the phenomenon of somaclonal variability to get the best mutants, aploid cells are cultivated in vitro as immature pollen. The embryogenesis of aploid plants is induced ny properly dosing with vegetable hormones. This facilitates the recognition of mutations, because that the heterozygote can disguise its phenototype. Once recognised and isolated the changed plant the diploid state can be reestablished with clochicina, an alkaloid derived from the plant of the autumn crocus that binds it to the molecules of [tubulina], thereby preventing the formation of the fused mitotic and therefore the [citodieresi]. The [cromatidis] brothers separate it and they become homologous chromosome. This technique is known with the name of androgenesis. More than a hundred species of commercially valuable plants have been "improved" in this way, among which are tobacco, bowline, grapes, potatoes, rice, maize and wheat.

Mechanisms that produce variability

Changes in the [ploidia] of the cells in crop Owed to:

the origin of the fabric used for the [espianto], as farther away from the meristematic apexes, that much taller the dismal fraction of cells - and [octaploidi]; the effects of the process of crop same (lasted, hormones, nutritional limitations); three phenomena that occur during mitosis: [Endomitosis]:: the [cromatidis] brothers separate, but there is no formation of a fused one [citodieresi]; Endoreduplication:: the chromosome of the [interphase] suffer an extra-duplication; formation of a [plasmodio]:: there is no [citodieresis], [bi] - or [multinucleate] cells are formed; A high rate of these phenomena doesn't correspond to a high percentage of cells or so-called polyploids. This is due to diploid selection: in a mixed population of cells with different ploidies, the diploids preserve their potential [organogenetico] better than the polyploids, probably because of an increased ability to form meristems.

artificial rearrangement of tissue layers A lot of horticultural plants are chimeras [PERICLINALI], that is they have suffered a mutation in a meristem cell and this has given origin to a different layer from the preceding and from the following one. The cells of the meristems in fact can divide anticlinally (perpendicularly to the surface) or periclinally (in parallel); if a continuous cell to divide itself periclinally gives origin to a layer of fabric. These layers can mix during a rapid proliferation as that of the calluses (if you/he/she is used really that meristems). Therefore regenerated plants with these unforeseen event can contain a different artificial composition or even not to be more artificial.

structural changes in the sequence of the DNA They are induced by radiation and chemical substances but can be also spontaneous. The coarse alterations of the genome are the principal cause of somaclonal variability. We distinguish:

deletions inversions duplications transpositions point mutations These alterations inasmuch changes of ploidy, increase with the increase of the duration of the crop.

phenotypic epigenetic alterations They are temporary and reversing changes, but can persist for the whole life of the regenerated seedling. Common is the phenomenon of the rejuvenation/reinvigoration, especially in woody kind (gymnosperms), which leads to morphological differences, precocious flowering, increase in the formation of temporary roots and the vigor of the plant. The causes of these non-hereditable alterations are not known, but are probably induced by the environment of the crop.

In vitro selection With this method a plant resistant to illnesses, insects and environmental stress can be obtained. It implicates subjecting a calliform population to a proper selective pressure (such as the increase in small doses of herbicides) and the recovery of a varying line of cells that has developed resistance or tolerance to the stress. You can benefit of some speed in the propagation of the crop of the calluses to obtain a real natural selection (spontaneous mutations) or to get variability through the use of chemical or physical agents (induced mutations). The fields of research try to select resistant lines to salinity, cold, herbicides, to heavy metals, etc.

Cellular hybrids The genetic information contained in cells of different origin can be combined in a single nucleus through cellular fusion. Cellular fusion requires that some cells come into contact and it includes a brief destruction of the cellular membranes using chemical agents. When the reconstitution of the membranes happens, the adjacent cells can reform their membranes together producing a single hybrid cell. Initially the cell derived by fusion will contain two nucleuses (binucleated heterocariont), but after the cellular division the chromosomal outfits of the two cells are placed inside a single nucleus (sincariont). It is possible to perform the fusion on two types of cells that belong to the same kind (in this case we speak of inter-species hybrid) as well as on two types of cells belonging to different kinds. In the first case the hybrid cell will preserve the whole chromosomal order of the two initial cells, while in the second case the fused cell curtains to eliminate the chromosome belonging to one type of cell. The products of the fusion of more than two cells generally has scarce possibilities of survival. Any cell can be interbred with any other, without limits; very fanciful experiments have been made, in which man-mouse cells were obtained, animal-vegetal cells and even with microorganisms!

Isolating them, the hybrids obtained in this way can be propagated in crop a Cellular Line it Interbreeds, that more stable as the initial organisms were more similar. Subsequently to the fusion of the nucleuses, during the following cellular divisions, the genes of one of the two kinds are progressively eliminated.

This has allowed the study of the genetic expression in different cellular environments of the usual ones and above all of the organization of the genome: by checking what chromosome or fragment of chromosome has been lost, it is possible to make a genetic map of the chromosome that contains the location of every single gene.

Despite this obstruction to the formation of a completely hybrid plant, it can be seen that a fraction of the genes can be preserved with formation of the so-called Asymmetrical Hybrid. You get a plant with all the characteristics of a progenitor anymore some line of the other. In this way it is possible to transfer useful qualities even if the sexual hybridization doesn't allow it.

The other sexual hybridization is nothing but the pollination driven by man of a plant of variety or, when it is possible, of different kind. Thus they gather the positive qualities of different varieties, selecting then for different generations up to stability genotypical and fenotypical.